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This is a classic question. When I first asked it, I was very young (so young that I don't remember how old I was), and I don't remember if my dad had an answer. After a few years had passed and I asked the question again, I remember thinking that when I grew up and had kids of my own, I'd want to be able to answer all sorts of questions like this one. Maybe that's why so much of my formative years were devoted to trivia and why I started to write this website... anyway, onto the shortest explanation I can give.

Short answer: The sky is blue because of the way the light from the sun interacts with our atmosphere. Our atmosphere is composed of mostly nitrogen (79%) and oxygen (20%). These molecules scatter the blue wavelengths of light more than other wavelengths (which more or less pass through). The blue light from the sun bounces off all the nitrogen and oxygen molecules in the sky and enter our eyes, so we see a blue sky.

Longer answer: When light passes through a suspension of small particles (which our atmosphere is), it bounces off the particles. We can see this when we shine a laser through a smokey chamber or a flashlight in a dusty room. (Photographers take advantage of this when dust or moisture causes beams of sunlight to be visible; these are often refered to as God's rays or incorrectly identified as corpuscular rays.) This effect is called the Tyndall effect. A consequence of light bouncing off of these particles is that the light is scattered (deflected into various directions). What is really happening as light bounces off is that light energy is absorbed by electrons. The electron is momentarily excited and then releases the energy in a direction and sometimes at a different wavelength. The conditions in which this occurs determines the type of scattering that will occur. Raleigh scattering is the type of scattering that causes the sky to be blue. (The other major type of scattering, Thompson, is responsible for the corona. Thompson scattering is caused by the excitation of free electrons such as those found in ionized space around the sun. Because Thompson scattering is wavelength independant, the corona is white.) Raleigh scattering affects high frequency electromagnetic waves more than low frequency. Ultraviolet and blue light is scattered everywhere, while red and infrared pretty much continue in the same direction. The light from the sun, minus some blue, directly reaches the earth and imparts its energy as heat. The blue light is scattered throughout the atmosphere, illuminating the entire sky. If not for the Raleigh scattering, the sky would just be a black field with a bright white point as the sun (like going outdoors in the middle of nowhere and looking at a lit lantern. Without anything to bounce the light back to you, everything else except the lantern still looks black.)

When the sun is low to the horizon, the light travels through significantly more atmosphere than during the day. At these low angles, the Raleigh scattering can effectively eliminate the high frequency wavelengths, leaving us with red sunsets. However, with nitrogen and oxygen alone, the amount of scattering results in a light orange-yellow hue. It is with the aid of other particular matter (such as water vapor, dust, smog, etc.) that helps scatter more of the low and medium frequency wavelengths that produce the insanely red sunsets that places such as Los Angeles are famous for.

It should also be noted that Raleigh scattering scatters not just blue, but also violet and ultraviolet light. This leads to the question of why don't we see a violet sky? There are several reasons (including how the eye detects the visible light spectrum), but the predominant one is that higher frequency wavelengths are emitted at lower intensities. Imagine that an electron has absorbed a certain amount of energy. When it releases the energy as visible light, that amount of energy must be preserved. So, blue light, which is lower frequency than violet light, will be emitted with greater intensity (brightness) than the violet which is emitted with greater intensity than the ultraviolet.

Note: There's a lot of incorrect or partial information available in textbooks and on the web as to the cause of the sky's color. One such example is that several websites claim that the Tyndall effect scatters the light by wavelength, which is a misunderstanding of the definition of the Tyndall effect.

Last edited by Michael Chu on Tue Jul 19, 2005 3:29 am; edited 1 time in total

I recently bought a glass egg at the Science Museum of Virgina gift shop with interesting optical characteristics. It is blue by reflected light and orange by transmitted light. It is about the size of a chicken egg, and milky. There must be a suspension of particles of the right size to cause the Tyndall effect.

When my children were small, they learned not to ask questions like that. I would always over-answer. As a result, one is an artist, and the other a musician.

Agree with all said. Just being redundant in different words; if nature had given our eyesight a broader capacity to distinguish colors, the sky would really look violet, as it is to many creatures with that broader capacity.

Agree with all said. Just being redundant in different words; if nature had given our eyesight a broader capacity to distinguish colors, the sky would really look violet, as it is to many creatures with that broader capacity.

Humans can see violet, so why shouldn't we see that if those colors visible to us were there? I think you are mistaken.

If we could see far IR or UV, it might look different to us, but you can't say that far IR would look anything like red and you also couldn't say that UV would look anything like violet. Another case in point, you can't tell me what color your favorite FM radio station signal looks like nor can you tell me what X-rays look like. If your dog could speak English and understand you in regular conversation, you still couldn't tell it what orange looked like, because dogs are colorblind. To say what a color normally invisible to humans looks like, is like trying to tell a person blind at birth what green looks like. While I am not a retinologist or in the ophthalmic field, I believe knowing what a radiation that we do not have our own bodily sensors for, is unknowable (and I used that term with great care) with the present science. Colorizing it so we can "imagine it" is not the same thing. We do not know what it looks like to an insect, for example to see UV. Being able to perceive a radiation is not the same as being able to say what the brain's interpretation of it is when comparing different animals.

My 12 year old neice asked for my favorite color - my reply was "microwave". Any sane adult would have left me for insane and walked away, but children are wonderfully curious. I got to explain to her how the big plastic box that heats food while itself called a microwave is really a microwave oven, a box that uses a form of light called microwaves to excite water and thus heat it up. I'm pretty sure she was still fairly confused, but she seemed to get a lot more out of the conversation than a 22yr old would have.

My 12 year old neice asked for my favorite color - my reply was "microwave". Any sane adult would have left me for insane and walked away, but children are wonderfully curious. I got to explain to her how the big plastic box that heats food while itself called a microwave is really a microwave oven, a box that uses a form of light called microwaves to excite water and thus heat it up. I'm pretty sure she was still fairly confused, but she seemed to get a lot more out of the conversation than a 22yr old would have.

In college (last year), my engineering buddies and I were into home brewing and at one point we made a "680nm Ale", which to any person who knows their beers and their spectrum, would know that it was our take on a Red Ale.

Not many other people (outside of school that is) got the joke.

We also had "Dijkstra's Ale-gorithm", "root Beer" (as in the user), and "Hello World Stout" (which was out first beer).

I bet the sky over MARS is also blue but NASA doesnt want us to know that!!

That's what I thought too:)
No, seriously the sky is blue because of the atmosphere which causes a unique refraction of light that reaches our eyes.
The sky is sometimes red and even violet, for example at sunset.